This invention relates to a method for forming composite dielectrics particularly for use in silicon integrated circuit processes and to integrated circuits incorporating such dielectric layers.
Development of micron and submicron technology for VLSI circuits is marked by a rapid increase in development of improved techniques for the formation of very thin (less than 20 nanometers) dielectrics. Thermally grown silicon dioxide is currently the most well known and commonly used dielectric. However, very thin silicon dioxide layers suffer from such problems as high defect density, high probability of low-field breakdown, poor masking qualities against dopant diffusion, and anneal-dependent dielectric constant. The shortcomings of silicon dioxide are detailed by Lai in"Semiconductor Silicon 1981", Electrochemical Society, 416 (1981). These problems are greatly magnified when thin silicon dioxide"interpoly" layers are utilized as the dielectric medium between layers of polysilicon used in certain types of VLSI chip, Heimann et al, Journal of Applied Physics, Volume 53, 6240 (1982).
A thermally grown silicon nitride layer appears to be a suitable candidate for eliminating some of the above-mentioned problems, Nemetz et al, Solid State Technology, Volume 26, 79 (1983), and various techniques for thermal nitride formation, mainly in an ammonia ambient, are known, Murarka, Journal of Electrochemical Society, Volume 126, 996 (1979) and Ito et al, IEEE Transactions on Electron Devices, Volume 29, 498 (1982). For current VLSI technology, nitride dielectric layers for device gates and between layers of polysilicon are required to be of the order of 7 to 10 nanometers thick. However, a single dry nitrogen nitridation at 1000 degrees centigrade is self-limiting to less than 5 nanometers, Wu et al, Journal of Electrochemical Society, Volume 129, 1559 (1982).